Carbon has four known general structures including diamond, graphite, fullerene and carbon nanotubes. Crystalline structure refers to the lattice arrangement of atoms. Carbon nanotubes refer to tubular structures grown with a single wall or multi-wall, which can be thought of as a rolled up sheet formed of a plurality of hexagons, the sheet formed by combining each carbon atom thereof with three neighboring carbon atoms. The carbon nanotubes have a diameter on the order of a few angstroms to a few hundred nanometers. Carbon nanotubes can function as either an electrical conductor, similar to a metal, or a semiconductor, according to the orientation of the hexagonal carbon atom lattice relative to the tube axis and the diameter of the tubes.
Originally, carbon nanotubes were produced by an arc discharge between two graphite rods as reported in an article entitled “Helical Microtubules of Graphitic Carbon” (Nature, Vol. 354, Nov. 7, 1991, pp. 56-58) by Sumio lijima. This technique produced mostly multiwall carbon nanotubes. A method of producing mostly single wall carbon nanotubes was subsequently discovered and reported D. S. Bethune and co-workers (Nature, Vol. 363, pp. 605 (1993)).
There exist numerous applications for optically transparent, electrically conducting films. Since, as produced, single wall carbon nanotubes are known to contain a substantial fraction of intrinsically metallic nanotubes (typically about ⅓), nanotube films could be useful in such applications, provided the films were optically transparent, possessed uniform optical density across their area and possessed good electrical conductivity throughout the film. Optical transparency requires the films to be made sufficiently thin. Uniform optical density across an optical aperture requires the nanotubes to be homogeneously distributed throughout the film. Finally, good electrical conductivity throughout the film requires sufficient nanotube-nanotube overlap throughout the film.
The principal problem in producing nanotube films which meet these requirements of thinness, homogeneity and good intertube contact is the lack of solubility of the nanotubes in any known evaporable solvent. Given such a solvent, the nanotubes could simply be dissolved in a dilute concentration and then cast or sprayed in a thin uniform layer on a surface, leaving behind the desired transparent nanotube layer once the solvent evaporates. Because no such solvent for the nanotubes is known, if a deposition is attempted with nanotubes dispersed (e.g. by ultrasonication) in a solvent such as ethanol, inhomogeneous clumps of nanotubes result over the area of the deposited region.
Nanotubes can be uniformly suspended in solutions with the aid of stabilizing agents, such as surfactants and polymers, or by chemical modification of the nanotube sidewalls. However, stabilizing agents interfere with the required electrical continuity of the nanotube film. Stabilizing agents are generally electrical insulators. Once the solvent is evaporated, both the nanotubes and the stabilizing agent remain, the stabilizing agents interfering with the intertube electrical contact. In the case of chemical modification of the nanotube sidewall, the electrical conductivity of the nanotubes themselves is degraded.
As a result, while thin and reasonably transparent films of nanotubes have been produced for certain scientific purposes, such as for recording optical transmission spectra, these films have not provided sufficient electrical conductivity necessary for applications requiring films which provide both high electrical conductivity and optical transparency, such as for optically transparent electrodes.